As a railway worker you are incorrect yet again yes there is legislation! Basically it’s that every train that crosses the viaduct in regular passenger service must stop at Stockport! It was a condition of the viaduct being built and has never changed! Whether if someone wanted to have a service not stopping at Stockport could challenge it I have no idea!
As for the metro it is rather a waste of money as others have said! It will extend in from east Didsbury and eventually it may well end up extending to join at the old junction just past bredbury tunnels and run to rose hill but that will only happen if “tramtrain” is deemed a success after its trials between Meadowhall and Rotherham